为什么天空有时会变成紫色?

为什么天空有时会变成紫色?

A blue sky is a good sky. It's a reassuring sight that promises clear weather and bright days.

蓝色的天空是一个好天气。这是令人安心的景象，预示着晴朗的天气和晴朗的日子。

When you see purple skies, it's because light has scattered in a different way than usual. (Photo: JhonnierAndrey/Wikimedia Commons)

But what in the world does a purple sky mean? It's not uncommon to spot purple skies during sunsets or sunrises. Sometimes they even appear after hurricanes, as they did in Florida following Hurricane Michael.

但是紫色的天空到底是什么意思呢?在日落或日出时看到紫色的天空并不罕见。有时，它们甚至会在飓风过后出现，就像在飓风迈克尔之后的佛罗里达州一样。

But what causes them?

是什么引起的呢?

Seeing Blue

蓝色的天空

That's certainly a good view. (Photo: Juanedc/Wikimedia Commons)

To understand why the sky is sometimes purple, it's helpful to first understand why the sky is blue.

要理解为什么天空有时是紫色的，首先理解为什么天空是蓝色的是有帮助的。

Light is white, but when you put it through a prism, you see all the different colored light waves in the spectrum: red, orange, yellow, green, blue, indigo, and violet. Light travels in waves, sometimes in short dippy ones and other times in long lines with lots of peaks.

光是白色的，但是当你把它通过棱镜时，你会看到光谱中所有不同颜色的光波:红、橙、黄、绿、蓝、靛、紫。光以波的形式传播，有时是短而模糊的，有时则是有许多峰的长波。

Light will travel in a straight line unless something gets in its way, like a prism or molecules in the atmosphere.

光会直线传播，除非有东西挡住它，比如棱镜或大气中的分子。

Gases and particles in the atmosphere scatter light, and since blue travels on shorter, smaller waves, it causes charged particles to move faster, scattering more light. So we see more blue than we do red because blue gets particles worked up more than the other colors. Our eyes are also a bit more sensitive to blue light, which helps.

大气中的气体和粒子会散射光，由于蓝色的传播波更短更小，它会导致带电粒子运动得更快，散射出更多的光。所以我们看到的蓝色比红色更多，因为蓝色比其他颜色更能激发粒子。我们的眼睛对蓝光更敏感，这也有帮助。

All About the Angles

都是关于角度

Even the landing lights of Osaka Kansai International Airport can't detract from that red-orange sunset. (Photo: Ken H/Wikimedia Commons)

Violet is always there, but we don't detect it as well as we do blue, so blue wins the light wave battle for our eyes. The right conditions have to be met for purple, or violet, to be visible.

紫罗兰色一直存在，但我们对它的探测不如对蓝色的探测，所以蓝色赢得了我们眼睛的光波大战。要想看到紫色，必须满足适当的条件。

In some cases, it's a matter of where the sun is coming in at a certain angle. Some of those colors that get blocked out by blue, like yellow, red and orange, appear a lot during sunrises and sunsets for this reason.

在某些情况下，这是太阳以特定角度从哪里进来的问题。因此，在日出和日落的时候，会经常出现一些被蓝色遮挡的颜色，比如黄色、红色和橙色。

Purple Skies

紫色的天空

That's some purple power in Ludwigsburg, Germany. (Photo: Sascha Schuermann/AFP/Getty Images)

But other factors can come into play that can jumble up the light waves and the particles even more.

但其他因素也会起作用，使光波和粒子更加混乱。

According to Sarah Keith-Lucas from BBC Weather, "dust, pollution, water droplets and cloud formations" can influence the colors of the sky, too. Occasionally, pink and purple will appear more often than red and orange due in part to "the optical illusion of the pink wavelengths lighting up the base of the cloud (due to the low angle of the sun's rays), and these pink clouds superimposed on a dark blue sky. The combination of pink and dark blue can make the sky appear a deep purple."

据BBC气象频道的莎拉·基斯-卢卡斯说，“灰尘、污染、水滴和云的形成”也会影响天空的颜色。偶尔，粉色和紫色会比红色和橙色出现得更频繁，部分原因是“粉色波长照亮云的底部的光学错觉(由于太阳光线的低角度)，这些粉红色的云叠加在深蓝色的天空上。”粉色和深蓝色的组合可以让天空呈现出深紫色。”

In the case of Hurricane Michael, water droplets, a setting sun and low cloud cover played a part in creating a purple sky after Michael had passed.

以“迈克尔”飓风为例，水滴、落日和低云层在“迈克尔”飓风经过后形成了一片紫色的天空。

In the end, getting those purple hues is about having just the right conditions happen at the right time.

总之，要想获得这些紫色的色调，需要在合适的时间、合适的条件下。